(MFC) S3 Resume Download

Suppose an S3 download of a very large file failed for some reason and you have a partial file on disk. Rather than restart the entire download, you wish to download the remaining portion. This example demonstrates how to finish a previously failed download.

Note: This example requires Chilkat v9.5.0.83 or above.

Chilkat C/C++ Library Downloads MS Visual C/C++ Libs

See Also: Using MFC CString in Chilkat

#include <CkRest.h>
#include <CkAuthAws.h>
#include <CkFileAccess.h>
#include <CkStringBuilder.h>
#include <CkStream.h>

void ChilkatSample(void)
    {
    CkString strOut;

    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    CkRest rest;

    // Connect to the Amazon AWS REST server.
    bool bTls = true;
    int port = 443;
    bool bAutoReconnect = true;
    bool success = rest.Connect("s3.amazonaws.com",port,bTls,bAutoReconnect);

    // ---------------------------------------------------------------------------
    // Important: For buckets created in regions outside us-east-1,
    // there are three important changes that need to be made.
    // See Working with S3 Buckets in Non-us-east-1 Regions for the details.
    // ---------------------------------------------------------------------------

    // Provide AWS credentials for the REST call.
    CkAuthAws authAws;
    authAws.put_AccessKey("AWS_ACCESS_KEY");
    authAws.put_SecretKey("AWS_SECRET_KEY");
    authAws.put_ServiceName("s3");
    success = rest.SetAuthAws(authAws);

    // Set the bucket name via the HOST header.
    // In this case, the bucket name is "chilkat100".
    rest.put_Host("chilkat100.s3.amazonaws.com");

    // We want to continue downloading a file.
    // The relative local filepath of our previously partially downoaded file is: qa_output/hamlet.xml
    // Let's find out how many bytes are already downloaded.
    CkFileAccess fac;
    const char *localFilepath = "qa_output/hamlet.xml";
    // Note: The FileSize method returns a signed 32-bit integer.  If the file is potentially larger than 2GB, call FileSizeStr instead to return
    // the size of the file as a string, then convert to an integer value.
    int sz = fac.FileSize(localFilepath);
    if (sz > 0) {
        strOut.append("sz = ");
        strOut.appendInt(sz);
        strOut.append("\r\n");

        // If the sz equals 42375 bytes, then we want to add a Range header that looks like this:
        // Range: bytes=42375-
        CkStringBuilder sbRange;
        sbRange.Append("bytes=");
        sbRange.AppendInt(sz);
        sbRange.Append("-");
        rest.AddHeader("Range",sbRange.getAsString());
        strOut.append("Added Range: ");
        strOut.append(sbRange.getAsString());
        strOut.append("\r\n");
    }

    // Send the request to download the remainder of the file.
    success = rest.SendReqNoBody("GET","/hamlet.xml");
    if (success != true) {
        strOut.append(rest.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    // Read the response header.
    int responseStatusCode = rest.ReadResponseHeader();
    if (responseStatusCode < 0) {
        strOut.append(rest.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    strOut.append("Response status code = ");
    strOut.appendInt(responseStatusCode);
    strOut.append("\r\n");

    // We expect a 200 or 206 response status if the file data is coming.
    // Otherwise, we'll get a string response body with an error message(or no response body).
    if ((responseStatusCode == 200) || (responseStatusCode == 206)) {

        CkStream bodyStream;

        // The stream's sink will be a file.
        // We will append to the file..
        bodyStream.put_SinkFile(localFilepath);

        // Indicate that we wish to append to the output file.
        // The SinkFileAppend property was added in Chilkat v9.50.83
        bodyStream.put_SinkFileAppend(true);

        // Read the response body to the stream.  Given that we've
        // set the stream's sink to a file, it will stream directly
        // to the file.
        success = rest.ReadRespBodyStream(bodyStream,true);
        if (success != true) {
            strOut.append(rest.lastErrorText());
            strOut.append("\r\n");
            SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
            return;
        }

        strOut.append("Successfully downloaded the file.");
        strOut.append("\r\n");

    }
    else {
        const char *errResponse = rest.readRespBodyString();
        if (rest.get_LastMethodSuccess() != true) {
            strOut.append(rest.lastErrorText());
            strOut.append("\r\n");
        }
        else {
            strOut.append(errResponse);
            strOut.append("\r\n");
        }

    }



    SetDlgItemText(IDC_EDIT1,strOut.getUnicode());

    }